Optical recording film and process for production thereof

ABSTRACT

There are disclosed the following three types of optical recording film: 
     (1) an optical recording film comprising a gel having a network structure of an inorganic substance and a polymer which is from a photopolymerizable compound (A) and present in the network structure in the gel, said gel containing an optical recording-induced difference in the network structure. 
     (2) an optical recording film, which consists essentially of a porous gel having a network structure of an inorganic substance, the porous gel containing recording-induced porosity differences in the network structure. 
     (3) an optical recording film which consists essentially of a porous gel having a network structure of an inorganic substance or a gel obtained by densification of the porous gel, the porous gel or the gel having a recording-induced concavo-convex form on the surface thereof.

This is a divisional of Ser. No. 08/412,021, filed Mar. 28, 1995, nowallowed which is a divisional of Ser. No. 08/279,627, filed Jul. 25,1994, now abandoned, which is a continuation of Ser. No. 08/086,241,filed Jun. 30, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording film and a processfor the production thereof, and further to a film for optical recording,a process for the production thereof and a composition useful for theproduction of the film for optical recording.

2. Prior Art

Holograms are known which are classified into amplitude and phase types(refractive index modulation types) in respect of their recordingprinciples, into surface and volume types in respect of theirstructures, and into transmission (opposing direction) and reflection(same direction) types in respect of directions of illuminating lightand diffracted light in reconstruction. Among these, a volume phase typeis excellent particularly in optical characteristics such as diffractionefficiency. It has been already demonstrated that the maximumdiffraction efficiency of the volume phase type including thetransmission type and the reflection type can be theoretically 100percent. It is therefore considered that the volume phase type hologramcan be applied not only as a display hologram for recording images butalso as a variety of optical devices utilizing high diffractionefficiency, such as a grating device, an optical division device, anoptical coupling device and a laser beam scanning device.

The volume phase hologram of a reflection type in particular has intenseinterference activity and remarkable wavelength selectivity and canhence permit reconstruction in white light. Therefore, the volume phasehologram of a reflection type due to this feature and brightness of itsreconstructed image is attracting wide attention. Further, due to itshigh wavelength selectivity, it is put to practical use in some areassuch as head-up displays for an aircraft, an automobile and a vehicleand laser protection ophthalmic glasses.

The material for the volume phase type hologram has been selected fromphotosensitive materials for photograph typified by a silver halideemulsion and dichromated gelatin (DCG). The silver halide emulsion andDCG have so far been widely used since the former has high sensitivityand the latter has excellent optical properties such as high diffractionefficiency.

Since, however, DCG is poor in storage stability, it is always requiredto prepare DCG before exposure. Further, it requires complicated wetprocessing such as development and fixing after holographic exposure,and it also has a problem in that a hologram produced therefrom is stillinsufficient in light resistance and humidity resistance. These problemshinder commercial production of a hologram from DCG.

Silver halide emulsion has sufficient performance as a holographicmaterial as far as its sensitivity is concerned. However, the problemthereof is that it exhibits no high resolution due to its particulateproperties, that is, it is not properly used for recording interferencefringes having a high spatial frequency, and that the transmittancedecreases. Further, there is another problem in that it is poor in lightresistance when bleached for producing a phase type hologram. Moreover,it also requires important and essential but complicated wet processingsuch as development and fixing after holographic exposure as DCG does.

For overcoming the above problems of conventional materials for thevolume phase hologram, the development of a photopolymer is under way inrecent years. A photopolymer is excellent in storage stability and isessentially free of a resolution problem since it has no particulateproperties. Further, it is said that it is possible to improve lightresistance and humidity resistance by selecting its composition, and theproblem in reconstruction of recorded data is being solved.

The photopolymer is classified into a photocrosslinkable polymer and aphotopolymerizable polymer. The photocrosslinkable polymer includes apolymer whose molecule has a photocrosslinking functional group asdisclosed in Japanese Laid-open Pat. Publications Nos. 114029/1983 and211181/1983. When this polymer is used, the photocrosslinking proceedsaccording to light intensity distribution of interference fringes, andinterference fringes are recorded as a crosslinkage distribution. Inthis method, therefore, development as a post step is required forobtaining high diffraction efficiency.

Concerning the photopolymerizable polymer, there is a method using acombination of a so-called photopolymerizable monomer and a binderpolymer as is disclosed in U.S. Pat. Nos. 4,173,474, 4,535,041,4,942,112 and 4,963,471 and EP 324,482. In this method, thephotopolymerizable monomer is contained in an optical data-recordingmaterial, and the polymerization of the photopolymerizable monomerselectively proceeds according to light intensity distribution formed byholographic exposure, thereby to record the interference fringes. Inparticular, U.S. Pat. No. 4,963,471 uses a fluorine-containing polymerhaving a low refractive index as a binder in combination with a monomerhaving a high refractive index for obtaining high diffraction efficiencyin a reflection type phase hologram.

On the other hand, for using volume phase holograms of either atransmission type or a reflection type in a wider fields, it is desiredto impart not only excellent optical characteristics and easiness in apost step such as freedom of wet processing, but also high heatresistance and environment resistance. For achieving this object, it isexpected to develop a novel material for recording a hologram, which isbasically different from any prior materials.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical recordingfilm which can be a hologram excellent in diffraction efficiency,resolution and transmittance.

It is another object of the present invention to provide an opticalrecording film excellent in heat resistance and environment resistance.

It is another object of the present invention to provide a process forthe production of an optical recording film having the above excellentphysical properties.

It is another object of the present invention to provide a film foroptical recording to be used in the above production process, and aprocess for the production thereof.

It is further another object of the present invention to provide acomposition useful for producing the above film for optical recording.

According to the present invention, there is provided an opticalrecording film (hereinafter referred to as the first optical recordingfilm) comprising a gel having a network structure of an inorganicsubstance and a polymer which is from a photopolymerizable compound (A)and present in the network structure in the gel, said gel containing anoptical recording-induced difference in the network structure of thegel.

Further, there is provided a film for optical recording, which comprisesa gel having a network structure of an inorganic substance, aphotopolymerizable compound (A) and a photoinitiator (B), thephotopolymerizable compound (A) and the photoinitiator (B) beingcontained in the network structure.

Further, there is provided a process for the production of an opticalrecording film, which comprises recording optical data by exposing theabove film for optical recording to actinic radiation.

Further, there is provided a composition containing a photopolymerizablecompound (A), a photoinitiator (B), a metal compound (C) which iscrosslinkable by self-hydrolysis when in contact with water and thesubsequent polycondensation, a good solvent (D) for the above metalcompound, water (E) and a catalyst (F) for the hydrolysis of the abovemetal compound.

Further, there is provided a process for the production of a film foroptical recording, which comprises applying the above composition to asubstrate and drying the composition to form a solid-like filmcontaining a gel having a network structure of an inorganic substance, aphotopolymerizable compound (A) and a photoinitiator (B), thephotopolymerizable compound (A) and the photoinitiator (B) beingcontained in the network structure of the gel.

Further, there is provided an optical recording film (hereinafterreferred to as the second optical recording film) consisting essentiallyof a porous gel having a network structure of an inorganic substance,said porous gel containing optical recording-induced porositydifferences in the network structure.

Further, there is provided an optical recording film (hereinafterreferred to as the third optical recording film) consisting essentiallyof a porous gel having a network structure of an inorganic substance ora gel obtained by densification of the porous gel, the porous gel or thedensified gel having a optical recording-induced concavo-convex form onthe surface thereof.

Further, there is provided a process for the production of an opticalrecording film, which comprises applying the above composition to asubstrate to form a coating, drying the coating to form a film foroptical recording, irradiating the film with actinic radiation to recordoptical data, and removing the organic component contained in the film.

The above objects are achieved by these inventions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an optical system used for recordinga transmission type hologram (diffraction grating) in a film for opticalrecording, provided by the present invention, in Examples.

FIG. 2 is a schematic view showing an optical system used for recordinga reflection type hologram (diffraction grating) in a film for opticalrecording, provided by the present invention, in Examples.

DETAILED DESCRIPTION OF THE INVENTION

The first optical recording film provided by the present inventioncomprises a gel having a network structure of an inorganic substance anda polymer of a photopolymerizable compound (A) present in the networkstructure in the gel.

The inorganic substance includes metal oxides such as silicon oxide,titanium oxide, zirconium oxide and aluminum oxide. Of these, siliconoxide and titanium oxide are preferred.

The gel having a network of the above inorganic substance may have along chain bonded thereto, such as a polyalkylsiloxane chain, apolydialkyltitanoxane chain, a polydialkylzirconoxane chain or apolydialkylaluminoxane chain. The amount of the above chain based on themetal oxide is 50% by weight or less, preferably 30% by weight or less.

The photopolymerizable compound (A) can be selected fromphotopolymerizable monomers and oligomers containing at least onepolymerizable group typified by an acryloyl group, a methacryloyl group,a vinyl group and an allyl group.

Specific examples of the photopolymerizable monomers includemonofunctional acrylates such as tetrahydrofurfuryl acrylate,ethylcarbitol acrylate, dicylopentenyloxyethyl acrylate, phenylcarbitolacrylate, nonylphenoxyethyl acrylate, 2-hydroxy-3-phenoxypropylacrylate, ω-hydroxyhexanoyloxyethyl acrylate, acryloyloxyethylsuccinate, acryloyloxyethyl phthalate, phenyl acrylate, naphthylacrylate, tribromophenyl acrylate, phenoxyethyl acrylate,tribromophenyoxyethyl acrylate, benzyl acrylate, p-bromobenzyl acrylate,2,2-bis(4-methacryloxyethoxy-3,5-dibromophenyl) propane, isobornylacrylate, 2-ethylhexyl acrylate, lauryl acrylate,2,2,3,3-tetrafluoropropyl acrylate and acrylsilane and methacrylatescorresponding to these monofunctional acrylates; polyfunctionalacrylates such as 1,6-hexanediol diacrylate, butanediol diacrylate,ethylene oxide-modified tetrabromobisphenol A diacrylate,pentaerythritol triacrylate, trimethylolpropane triacrylate andbisphenol A diacrylate and methacrylates corresponding to thesepolyfunctional acrylates; vinyl compounds such as styrene,p-chlorostyrene, divinylbenzene, vinyl acetate, acrylonitrile,N-vinylpyrrolidone, vinylnaphthalene, N-vinylcarbazole and vinylsilane;and allyl compounds such as diethylene glycol bisallyl carbonate,triallyl isocyanurate, diallylidene pentaerythritol, diallyl phthalateand diallyl isophthalate.

Specific examples of the photopolymerizable oligomers include oligomersof the above photopolymerizable monomers, polyfunctional oligoacrylatessuch as urethane acrylate oligomer, epoxy acrylate oligomer, polyesteracrylate oligomer, polyol polyacrylate, modified polyol polyacrylate andpolyacrylate having an isocyanurate acid skeleton, and methacrylatescorresponding to these acrylates.

The above urethane acrylate oligomer includes those formed by anaddition reaction of polyisocynate, 2-hydroxyalkyl (meth)acrylate andpolyol. The polyisocyanate includes toluene diisocyanate, isophoronediisocyanate, trimethylhexamethylene diisocyanate and hexamethylenediisocyanate. The polyol includes polyether polyols such as polyethyleneglycol, polypropylene glycol and polytetramethylene glycol, polyesterpolyol, polycarbonate polyol and polysiloxane polyol.

The polymer of the photopolymerizable compound (A) may be a homopolymeror a copolymer.

The amount of the polymer per 100 parts by weight of the gel ispreferably 30 to 1,000 parts by weight, more preferably 100 to 800 partsby weight.

In the first optical recording film of the present invention, therefractive index of the inorganic substance and the refractive index ofthe polymer from the photopolymerizable compound (A) differ from eachother, and the difference between the refractive indices is preferablyat least 0.01, more preferably at least 0.03. The maximum difference isabout 1.0. For example, when the network of the inorganic substance isformed from SiO₂ having a low refractive index, it is preferred to use apolymer having a high refractive index in combination. When the networkis formed from TiO₂ having a high refractive index, it is preferred touse a polymer having a low refractive index in combination. Thepreferred combination of the inorganic substance with the polymer is acombination of a 2-hydroxy-3-phenoxypropyl acrylate homopolymer(refractive index 1.555) with SiO₂ (refractive index 1.46) and acombination of a 2-hydroxypropyl acrylate homopolymer (refractive index1.52) with TiO₂ (refractive index 2.40).

A network structure difference is present in the gel constituting thefirst optical recording film of the present invention, and opticalrecording is based on the difference. For example, the gel containscoarse network portions and dense network portions, and opticalrecording is based on this coarse-dense distribution. The presence ofcoarse and dense network portions in the gel corresponds to the presenceof portions which have different refractive indices in the gel.

When the above optical recording film is a volume phase hologram, thecoarse and dense network distribution is formed according to lightintensity distribution of interference fringes of coherent light. Thenetwork is coarsely present in a portion where light intensity is high,and it is densely present in a portion where light intensity is low.Thus, the coarseness-denseness is correlative to variation of therefractive index to cause refractive Index modulation.

Depending upon methods of recording optical data, the above opticalrecording film can be any one of a transmission type hologram and areflection type hologram.

The above first optical recording film of the present invention can beobtained by exposing a film for optical recording to actinic radiation,the film comprising a gel having a network structure of an inorganicsubstance, a photopolymerizable compound (A) and a photoinitiator (B)and the photopolymerizable compound (A) and the photoinitiator (B) beingcontained in the network structure of the gel.

As the inorganic substance constituting the gel of the film for opticalrecording, those described regarding the above optical recording filmincluding its preferred embodiments can be applied. And, the inorganicsubstance has a three-dimensional network structure, and forms a gel, inthe film for optical recording.

The photopolymerizable compound (A) is as described before. Further, itis preferred to combine the inorganic substance and thephotopolymerizable compound (A) such that the refractive index of theinorganic substance and the refractive index of a polymer from thephotopolymerizable compound (A) differ from each other by at least 0.01,particularly by at least 0.03.

Specific examples of the photoinitiator (B) include cycliccis-α-dicarbonyl compounds such as 2,3-bornanedione (camphorquinone),2,2,5,5-tetramethyltetrahydro-3,4-furoic acid(imidazoletrione),benzophenones such as 3,3′,4,4′-tetra-(t-butylperoxycarbonyl)benzophenone, ketones such asdiacetyl, benzyl, Michler's ketone, diethoxyacetophenone,2-hydroxy-2-methylpropiophenone and 1-hydroxycyclohexylphenylketone,peroxides such as benzoyl peroxide and di-tert-butyl peroxide, azocompounds such as allyldiazonium salt, aromatic carboxylic acids such asN-phenylglycine, xanthenes such as 2-chlorothioxanthone and2,4-diethylthioxanthone, diallyliodonium salts, triallylsulfonium salts,triphenylalkyl borate, iron-allene complex, bisimidazoles, polyhalogencompounds, phenylisoxazolone, benzoin ethyl ether, benzyldimethyl ketaland mixtures of these.

The film for optical recording, provided by the present invention, maycontain any one of photoinitiator aids such as amines, thiols andp-toluenesulfonic acid.

The film for optical recording, provided by the present invention, mayfurther contain a sensitizer such as a dyestuff for causing thepolymerization effectively. When visible light is used as the actinicradiation, the dyestuff is selected from those having absorption in aregion of visible light. Specific examples of the dyestuff includeMethylene Blue, Acridine Orange, thioflavin, ketocoumarin, ErythrosineC, Eosine Y, merocyanine, phthalocyanine and porphyrin. These dyestuffsmay be used alone or in combination.

The film for optical recording, provided by the present invention, mayfurther contain an improver (plasticizer) for improving the mobility ofthe photopolymerizable compound (A). The improver includes triethyleneglycol dicaprylate, triethylene glycol diacetate, triethylene glycoldipropionate, glycerin tributylate, tetraethylene glycol diheptanoate,diethyl adipate, diethyl sebacate and tributyl phosphate. The amount ofthe improver per 100 parts by weight of the photopolymerizable compound(A) is preferably approximately 0.1 to 10 parts by weight.

The film for optical recording, provided by the present invention,preferably contains 30 to 1,000 parts by weight of thephotopolymerizable compound (A) and the 0.01 to 30 parts by weight ofthe photoinitiator (B) per 100 parts by weight of the gel.

The above film for optical recording can be obtained by applying acomposition containing the photopolymerizable compound (A), thephotoinitiator (B), a metal compound (C) which is crosslinkable byhydrolysis when in contact with water and the subsequentpolycondensation, a good solvent (D) for the above metal compound, water(E) and a catalyst (F) for the hydrolysis of the above metal compound toa substrate, and drying the coating.

The above metal compound (C) which is crosslinkable by hydrolysis whenin contact with water and the subsequent polycondensation includesalkoxide, metal salts such as carboxylate, halide and nitrate, and metalcomplex such as acetylacetonate of silicon, titanium, zirconium oraluminum. Of these metal compounds, preferred are metal alkoxides havingan alkoxy group having 1 to 4 carbon atoms such as methoxy, ethoxy,n-propoxy, iso-propoxy, n-butoxy, sec-butoxy, iso-butoxy andtert-butoxy.

Specific examples of the metal alkoxide preferably includetetraethoxysilane, tetramethoxysilane, tetrabutoxysilane, titaniumtetraisopropoxide, titanium tetrabutoxide, zirconium tetramethoxide,zirconium tetrabutoxide, aluminum triethoxide and aluminum triethoxide.These metal alkoxides may be used alone or in combination.

A silicon compound having a group which reacts with the hydrolyzate fromthe metal compound (C) to allow a straight molecular chain to bond tothe gel may be used in combination with the metal compound (C). Thesilicon compound Includes silanol-terminated polydialkylsiloxane,epoxysilane and aminosilane. The silicon compound bonds to the gelformed from the metal compound (C) to impart the gel with flexibility.The amount of the silicon compound based on the metal compound (C) ispreferably 50% by weight or less, more preferably 30% by weight or less.

The above metal compound (C) undergoes hydrolysis when in contact withwater in a solution and undergoes polycondensation and crosslinking toform a gel. The good solvent (D) for dissolving the metal compound (C)includes alcohols having 1 to 4 carbon atoms such as methanol, ethanol,propanol and butanol.

The catalyst (F) for the hydrolysis of the metal compound (C) includes asolution of acid such as hydrochloric acid, sulfuric acid, nitric acidor acetic acid and a solution of a base such as sodium hydroxide,potassium hydroxide or ammonia. The concentration of the acid or base ispreferably 0.001 to 20 N.

In the above composition, provided by the present invention, the amounts(% by weight) of the components (A) to (F), when the total amount of thecomponents is 100% by weight, are preferably as follows.

Photopolymerizable compound (A)   10-80 Photoinitiator (B) 0.05-30 Metalcompound (C)   5-90 Solvent (D)   5-90 Water (E) 0.01-30 Catalyst (F)0.05-30

The above composition may contain the foregoing photosensitizer and theforegoing improver for improving the mobility of the photopolymerizablecompound (A) as required. The amount of each of the photosensitizer andthe improver based on the total amount of the components (A) to (F) is0.01 to 10% by weight. Further, the above composition may contain aleveling agent for obtaining a coating having a smooth surface.

When the photopolymerizable compound (A) is a solid or highly viscous atroom temperature, a solvent may be used for homogeneously dissolving thephotopolymerizable compound (A) in a solution of the composition.Further, a solvent may be used for dissolving the photoinitiator and thephotosensitizer, as required. The solvent is properly selected frommethanol, ethanol, isopropanol, toluene, dioxane, chloroform,dichloromethane, methylene chloride, tetrahydrofuran and the like. Thesesolvents can also work as the good solvent (D) for the metal compound.For example, isopropanol is a good solvent for both2-hydroxy-3-phenoxyhexyl acrylate (solid) and tetraethoxysilane.

The film for optical recording is obtained by coating the abovecomposition in a solution form on a smooth surface of a substrate suchas a glass plate or a silicon substrate, and drying the coating.

As the method for the above coating, there can be employable a varietyof methods such as a spin coating method, a dip coating method, a barcoating method, a flow coating (curtain coating) method and a methodusing a doctor blade or an applicator.

The coating is dried at room temperature or under heat at a temperaturebetween 20 and 60° C., optionally under reduced pressure, for 1 minuteto several days, whereby volatiles such as the solvent, water, catalyst,etc., are evaporated from the coating. Further, the metal compound (C)undergoes hydrolysis and the subsequent polycondensation to becrosslinked, whereby a gel having a network structure of the inorganicsubstance is formed, and at the same time, the network structurecontains the photopolymerizable compound (A), the photoinitiator (B) andoptionally the photosensitizer, etc.

As described above, the solid-like film for optical recording is formedon the smooth surface of the substrate. In the above drying procedure,the volatiles are not completely removed. However, if the content of theresidual volatiles is up to about 30% by weight, a substantiallysolid-like film can be obtained and can be used as the film for opticalrecording. When the residual solvent is the good solvent for thephotopolymerizable compound (A), it works as an improver for thephotopolymerizable compound (A) in mobility, and advantageouslyfunctions in the polymerization which follows the exposure.

The thickness of the film for optical recording is generally 0.01 to 100μm, preferably 1 to 30 μm. It is preferred to cover the surface of theabove film with a transparent resin film or glass sheet. When thesurface of the film is so-covered, the polymerization proceeds smoothlyafter the irradiation with actinic radiation, since the activity ofoxygen inhibiting the polymerization is prevented.

The film for optical recording is exposed to actinic radiation to givean optical recording film. The term “radiation” is used in a broad senseincluding ultraviolet light, visible light, infrared light, electronbeam, ion beam, and the like, and the term “actinic radiation” refers toradiation capable of causing a polymerization reaction of thephotopolymerizable compound (A).

A hologram is formed from the film for optical recording film by a knownmethod in which the film for optical recording is exposed tointerference fringes caused by a coherent radiation. A laser isgenerally used as a coherent-light source. The exposure can be carriedout by a method using a known holographic exposure optical system, whichmethod is generally called a two beam interference exposure method.Laser light from a laser oscillator is separated to two collimated beamsor diffused beams by means of a beam splitter, a beam expander or acollimator lens. One beam as reference light is projected on the filmfor optical recording, and the other beam is projected, for example, onan object when an image of the object is recorded, and light reflectedfrom the object is projected on the film as an object light. Thereference light and the object light form interference fringes, and theinterference fringes are recorded in the film by refractive indexmodulation to give an optical recording film (hologram).

When the above two beams are projected in one (same) direction from oneside, a transmission type hologram is obtained. When the two beams areprojected in opposite directions from two sides, a reflection typehologram is obtained. Depending upon the intensity of radiation and anarea on which an optical data is recorded, the time for irradiation withcoherent radiation for holographic exposure varies and is generally 0.1second to 30 minutes, and the exposure is carried out up to a totalexposure dose of 0.1 to 1,000 mJ/cm².

The process of refractive index modulation by exposure to interferencefringes is as follows. In an unexposed film for optical recording,first, the photopolymerizable compound (A) is homogeneously contained inthe network of the inorganic substance uniformly formed in the entiretyof the film. When the film is exposed to interference fringes, thepolymerization is selectively initiated according to a light intensitydistribution within the film. That is, the polymerization starts at aportion where the light intensity is high, and photopolymerizablecompound (A) is accordingly consumed. Therefore, photopolymerizablecompound (A) is supplied from a portion where the light intensity is lowto a portion where the light intensity is high, and the polymerizationis further promoted. During the polymerization, part of the network ofthe inorganic substance initially present in a high light intensityportion is pushed away into an adjacent low light intensity portion by apolymer whose volume has grown due to photopolymerizable compound (A)supplied from a low light intensity portion. In this case, the movementof the photopolymerizable compound (A) is facilitated in the presence ofa mobility improver.

As a result, in an optical recording film obtained by the above process,a region where the light intensity has been high, a polymer fromphotopolymerizable compound (A) is densely present and the network ofthe inorganic substance is coarsely present and a region where the lightintensity has been low, a polymer from photopolymerizable monomer (A) iscoarsely present and the network of the inorganic substance is denselypresent are formed, whereby regions having different compositions inaccordance with light intensity, i.e., regions whose refractive indicesare different are formed.

When the difference between the refractive index of the polymer and therefractive index of the inorganic substance forming the network islarge, the difference in refractive indices between the above tworegions is large, and a hologram having high diffraction efficiency isobtained.

For obtaining a film in which a specific pattern, e.g., a resist patternis recorded, a mask of a predetermined pattern formed from a substancenon-transparent to radiation is placed on a film for optical recording,and the film is exposed to radiation, e.g., from a high-pressure mercurylamp through the mask. In this case, too, the time for the exposure toactinic radiation is generally 0.1 second to 30 minutes, and the totalexposure dose is 0.1 to 1,000 mJ/cm².

Although the first optical recording film of the present invention canbe obtained by only exposing the film for optical recording to actinicradiation as described above, it is preferred that the so-obtainedoptical recording film is further subjected to a step of completing thepolymerization of unpolymerized photopolymerizable compound (A) andinactivating the photoinitiator and the photosensitizer.

The above step can be carried out by uniformly irradiating the entiresurface of the film, which has been exposed to actinic radiation, withradiation containing wavelength at which the photopolymerizable compound(A) can be photopolymerized. Under this irradiation the polymerizationof unpolymerized photopolymerizable compound (A) is promoted, and as aresult, the difference in refractive index increases. In a reflectiontype hologram in particular, therefore, the diffraction efficiencythereof increases. Further, the photoinitiator and the photosensitizercan be inactivated by the irradiation, whereby the optical recordingfilm is improved in durability such as heat resistance and humidityresistance. The above irradiation is generally carried out up to a totalirradiation dose of 10 to 10,000 mJ/cm².

The film which has been exposed to actinic radiation may be subjected toheat treatment at 60° C. or higher in place of, or after, the aboveuniform irradiation with radiation. Under the heat treatment, thepolymerization of unpolymerized photopolymerizable compound (A) in thefilm is completed, and as a result, the difference in refractive indexincreases, whereby the diffraction efficiency increases. Further, thesolvent is removed by gasification under the heat treatment. Therefore,the diffraction efficiency is improved, and the optical recording filmis also improved in heat resistance and humidity resistance. The aboveheat treatment is generally carried out between 60 and 200° C. for 1minute to 4 hours.

The first optical recording film produced by the above method can be asurface phase type hologram or a volume phase type hologram. When thisoptical recording film is a volume phase type hologram, the hologram isexcellent in diffraction efficiency, resolution and transmittance andtherefore can be used as optical devices such as display hologram, agrating, an optical division device and an optical coupling device.

The second optical recording film of the present invention consistsessentially of a porous gel having a network structure of an inorganicsubstance, and it is an optical recording film in which a networkportion having a high porosity and a network portion having a lowporosity are present in the porous gel, those different porosities beinginduced by an optical recording. The presence of network portions havingdifferent porosities in the porous gel corresponds to a presence ofportions having different refractive indices in the porous gel.

When the above second optical recording film is a surface phase typehologram or a volume phase type hologram, the difference in porosity isformed according to the light intensity distribution of interferencefringes of coherent light. A portion exposed to light having highintensity forms a network portion having a high porosity, and a portionexposed to light having low intensity forms a network portion having alow porosity. The network portion having a high porosity has a lowrefractive index, and the network portion having a low porosity has ahigh refractive index. Thus, the second optical recording film ismodulated in refractive index.

The third optical recording film of the present invention consistsessentially of a porous gel having a network structure of an inorganicsubstance or a gel obtained by densification of the porous gel, and itis an optical recording film in which an concavo-convex form is presenton the surface of the gel.

When the above optical recording film is a surface phase hologram or avolume phase hologram, a concavo-convex form is formed on the gelsurface according to the light intensity of interference fringes ofcoherent light. A portion exposed to light having high intensity forms aconcave portion and a portion exposed to light having low intensityforms a convex portion. Due to this concavo-convex form, the phase ismodulated.

The second and third optical recording films can be obtained by removingorganic components from the first optical recording film. In the firstoptical recording film of the present invention, the refractive index ofthe inorganic substance and the refractive index of the polymerpreferably differ from each other, while the difference in refractiveindex is not important for obtaining the second and third opticalrecording films of the present invention.

The above organic components include not only the polymer from thephotopolymerizable compound (A) but also residual components such asunpolymerized photopolymerizable compound (A), the photoinitiator, thedyestuff and solvents. The organic components can be removed, forexample, by a method in which the film is heated up to 200° C. orhigher. In this heat treatment, the organic components in the opticalrecording film are removed from the film by oxidation and decomposition,and portions from which the organic components have been removed remainas pores, in which gases such as air are present. The temperature forthe above heat treatment depends on the organic compounds to be removedsuch as the photopolymerizable compound (A) and a solvent. Forincreasing the denseness and mechanical strength of the opticalrecording film, it is preferred to heat the film up to hightemperatures.

Further, the organic components may be also removed from the firstoptical recording film by another method in which the organic componentsare oxidized and decomposed by means of ozone generated by irradiationwith ultraviolet light having a wavelength of about 184 nm or by amethod in which the organic components are eluted in a solvent. Thesemethods may be employed in combination.

In the above step of removing the organic components, the opticalrecording film is rendered porous, and the film shrinks in the thicknessdirection so that it has a thickness of approximately ½ to {fraction(1/20)}. This optical recording film would also shrink in the in-planedirection. Since, however, the film is held on the substrate, actually,it hardly shrinks in the in-plane direction. In this case, changes oftwo types occur.

The change of the first type is that the organic components are removedfrom a region rich with an organic polymer thereby to increase theporosity of the region. In contrast, a region rich with inorganicnetwork has a lower porosity than the region which has been rich with anorganic polymer. And, the portion having a lower porosity has a highapparent refractive index than the portion having a high porosity. Thatis, in this case, the light intensity distribution is recorded as aporosity distribution, whereby the second optical recording film of thepresent invention is obtained.

The change of the second type is that the porous film is furtherdensified by further heating the porous film after the change of thefirst type has occurred, whereby a region which has a high porosity andhas been rich with an organic polymer shrinks to a greater degree than aregion rich with an inorganic region. As a result of this change, thereis obtained the third optical recording film of the present invention onthe surface of which a concavo-convex form is formed. A portion exposedto light having high intensity forms a concave portion, and a portionexposed to light having low intensity forms a convex portion.

Differing depending upon the composition and thickness of the firstoptical recording film, the intensity distribution of light to which thefilm is exposed and heating temperature, the height of the aboveconcavo-convex form is generally 0.001 to 10 μm, preferably 0.1 to 5 μm.

The heating temperature and time required for forming the second orthird optical recording film differ depending upon kind and amount ofthe inorganic substance and organic components. These conditions can beexperimentally determined beforehand. In general, the heatingtemperature is between 200 and 1,200° C., and the heating time is 1minute to 5 hours.

When the heating temperature is extremely high, the porosity is reducedto zero in both higher porosity regions and lower porosity regions, anda concavo-convex surface alone is formed. Further, it is possible toform both a concavo-convex surface and a pore distribution. When boththe concavo-convex surface and pore distribution are formed, these workadditively to increase diffraction efficiency.

The second optical recording film can be any one of a transmission andreflection type hologram as well, while it is suitable as a transmissiontype hologram. The third optical recording film is used as atransmission type hologram.

The above second and third optical recording films are not onlyexcellent in diffraction efficiency, resolution and transmittance butalso excellent in heat resistance, environmental resistance and lightresistance when used as surface phase type holograms in particular.Therefore, they can be used as optical devices such as a grating, anoptical division device and an optical coupling device.

The present invention will be explained hereinafter by reference toExamples. However, the present invention shall not be limited thereto.

In the following Examples 1 to 5 and 7, transmission and reflection typeholograms (gratings) were produced by means of holographic exposureoptical systems shown in FIGS. 1 and 2 for easily determining theperformances of optical recording films.

FIG. 1 shows an optical system for recording a transmission typehologram (grating), in which numeral 1 indicates an argon ion laserhaving a total output of 4 W and being to emit light having a wavelengthof 514.5 nm, numeral 2 indicates a shutter, numeral 3 indicates aspatial filter, numeral 4 indicates a collimator lens, numeral 5indicates a beam splitter, numerals 6 and 6′ indicate mirrors, andnumeral 7 indicates a material for recording a hologram. In this case,the spatial frequency (fringe spacing) of interference fringes to beformed changes depending upon an incident angle (θ) of beam to thematerial 7.

FIG. 2 shows an optical system for recording a reflection type hologram(grating), in which numeral 11 indicates an argon ion laser which is thesame as that shown in FIG. 1, numeral 12 indicates a shutter, numeral 13indicates a spatial filter, numeral 14 indicates a collimator lens, andnumerals 16 and 16′ indicate mirrors. A mirror 19 is placed on the backside of a material 17 for recording a hologram, and interference fringesformed by light from the collimator lens 14 and light reflected from themirror 19 are recorded. In this case, a liquid 18 (xylene) for matchingthe refractive index is used between a substrate glass and the mirror19.

The abbreviations used in Examples stand for the following compounds.

TEOS: Tetraethoxysilane

PDMS: Silanol-terminated polydimethylsiloxane (molecular weight 1,700)

THF: Tetrahydrofuran

i-PA: Isopropyl alcohol

Ti(Oi-Pr)₄: Titanium tetraisopropoxide

HPPA: 2-Hydroxy-3-phenoxypropyl acrylate

HPA: 2-Hydroxypropyl acrylate

TMPTA: Trimethylolpropane triacrylate

TMPTA-EO6: Adduct of TMPTA with 6 ethylene oxide units BTTB: 3,3′,4,4′-Tetra-(tert-butylperoxycarbonyl) benzophenone (purity 50%, suppliedby Nippon Oil & Fats Co., Ltd)

KCD: 3,3′-Carbonylbis(7-diethylaminocoumarin) (supplied by NipponKankoh-shikiso Kenkusho Co., Ltd)

EBPA: Ethoxylate bisphenol A diacrylate

EXAMPLE 1

A solution 1 having the following composition and a solution 2 havingthe following composition were individually prepared. The solution 2containing a hydrolysis catalyst was added dropwise to the solution 1with stirring to prepare a homogeneous solution. Then, the so-preparedsolution was refluxed at 80° C. for 30 minutes to obtain a homogeneousmetal compound solution.

<Solution 1> TEOS: 45 g PDMS: 5 g THF: 10 cc i-PA: 15 cc <Solution 2>i-PA 20 cc H₂O 7.8 cc HCl (concentration 12N) 3.6 cc

Then, under a red safety lamp on, a photopolymerizable monomercomposition (solution 3) having the following composition (including aphotoinitiator and a dyestuff) was added to and mixed with, whilestirring, the above-obtained metal compound solution in such an amountthat the proportion of the solution 3 based on the metal compoundsolution was 20, 25 or 33% by weight to give three compositions(homogeneous solutions) for recording a hologram.

Incidentally, the solution 3 had been prepared by dissolving BTTB andKCD in a mixture of methylene chloride with methanol, and then adding aphotopolymerizable monomer.

<Solution 3> TMPTA-EO6 4.75 g TMPTA 0.25 g BTTB 1.00 g KCD 0.05 gMethylene chloride/methanol (95/5, weight 2.00 g ratio)

Under a safety lamp, the above three compositions were independentlycoated on 300×150×2 mm glass substrates with an applicator, and thenvolatiles such as i-PA, THF, etc. were evaporated by allowing the glasssubstrates to stand for about 10 hours to give three photosensitivelayers (films for optical recording) having a thickness of about 10 μm.Then, a 100 μm thick cover film of polyethylene terephthalate wasattached onto each photosensitive layer, and the resultant sets wererespectively cut to a size of 60×60 mm to give three photosensitivematerials each of which was composed of a laminate of glasssubstrate-photosensitive layer-polyethylene terephthalate film.

Then, in the optical system shown in FIG. 1, 514.5 nm light oscillatedfrom the argon ion laser was split into two collimated beams through thecollimator lens 4 and the beam splitter 5 and projected on each of theabove three photosensitive materials (laminate of glasssubstrate-photosensitive layer-polyethylene terephthalate film) at anangle of θ to expose each photosensitive material. The values of theangle θ were respectively 5°, 14° and 42°.

After the above interference exposure, the total surface of eachphotosensitive material was irradiated with a 30 W fluorescent lamp at adistance of 3 cm for about 20 minutes, and heat-treated at 100° C. for 2hours to complete the polymerization and fix it.

In the above procedures, a transmission type hologram (grating) wasrecorded in each of the three photosensitive layers by means ofinterference fringes having a spatial frequency of 170, 470 or 1,400line pairs/mm. In all the cases, the hologram was excellently recorded.Above all, in the photosensitive material having the photosensitivelayer prepared by adding 25% by weight of the monomer composition(solution 3) to the metal compound solution, particularly brightdiffracted light was observed. All the photosensitive materials showedhigh exposure sensitivity and the exposure energy was 30-50 mJ/cm².

When laser beam was projected on these transmission type holograms,diffracted light was observed in all the holograms as theoreticallyexpected. In particular, in the holograms having a spatial frequency of1,400 line pairs/mm to clearly show Bragg diffraction, bright firstorder diffracted light was observed.

The photosensitive material prepared by adding 33% by weight of themonomer composition to the metal compound solution had a diffractionefficiency of about 35%.

The gel (silicon oxide) constituting the holograms had a refractiveindex of 1.46, and the copolymer of photopolymerizable monomers had arefractive index of 1.51.

EXAMPLE 2

A photopolymerizable monomer composition having the followingcomposition including a photoinitiator and a deystuff (solution 4) wasadded to the same metal compound solution as that used in Example 1 insuch an amount that the proportion of the photopolymerizable monomercomposition based on the metal compound solution was 20, 25 or 33% byweight. The resultant mixtures were respectively stirred to give threehomogeneous solutions of compositions for optical recording.

<Solution 4> HPPA 4.75 g TMPTA 0.25 g BTTB 1.00 g KCD 0.05 g Methylenechloride/methanol (95/5, weight 2.00 g ratio)

Photosensitive materials having about 8 μm thick photosensitive layerswere prepared from the above three compositions for optical recording inthe same manner as in Example 1, a transmission type hologram (grating)having a spatial frequency of 170, 470 or 1,400 line pairs/mm wasrecorded in each of the photosensitive materials. In all the cases, thetransmission type hologram was as excellently recorded as in Example 1.

When laser beam was projected on these transmission type holograms,diffracted light was observed in all the holograms as theoreticallyexpected. In particular, in the hologram having a spatial frequency of1,400 line pairs/mm to clearly show Bragg diffraction was recorded,remarkably bright first order diffracted light was observed.

The gel (silicon oxide) constituting the holograms had a refractiveindex of 1.46, and the copolymer of photopolymerizable monomers had arefractive index of 1.55.

The brightness of diffracted light from the holograms was higher thanthat of diffracted light from the holograms in Example 1. That isbecause the used photopolymerizable monomer had a higher refractiveindex so that larger refractive index modulation was obtained. When thetransmission type holograms obtained in this Example were heat-treatedat 300° C. for 2 hours, the gratings retained the function of thegratings and showed high heat resistance. For comparison, a hologramcomposed of a wholly organic composition was heat-treated in the samemanner, and it was found that the function of the grating was completelylost.

EXAMPLE 3

A reflection type hologram was recorded in the same photosensitivematerial as the one obtained in Example 2, which was formed from thehomogeneous solution prepared by adding 33% by weight of thephotopolymerizable monomer composition (solution 4) to the same metalcompound solution as that of Example 1. FIG. 2 shows the exposure methodemployed in this Example. That is, a mirror 9 was placed toward thereverse side of the photosensitive material (laminate of glasssubstrate/photosensitive layer/polyethylene terephthalate) through arefractive index matching liquid (xylene) 18, and interference fringesformed by parallel beams obtained through a collimator lens 14 and lightreflected from the mirror were recorded.

After the above holographic exposure, the entire surface exposure andheat treatment were carried out. The diffraction efficiency was improvedstep after step, and after the heat treatment at 200° C., a maximumdiffraction efficiency value of 43% was obtained. Table 1 shows theresults. The exposure sensitivity was as high as 30-50 mJ/cm².

TABLE 1 Step Diffraction efficiency (%) After homographic 10.1 exposureAfter exposure of 21.1 entire surface After heat treatment 100° C. - 2hr. 25.9 200° C. - 2 hr. 43.0

EXAMPLE 4

A solution 5 having the following composition and a solution 6 havingthe following composition were individually prepared, and the solution 6containing a hydrolysis catalyst was added dropwise to the solution 5with stirring to prepare a metal compound solution.

<Solution 5> Ti(Oi-Pr)₄ 20 g i-PA 20 cc <Solution 6> i-PA 40 cc H₂O 0.5cc HCl (concentration: 12N) 2 cc

50 Parts by weight of the following photopolymerizable monomercomposition (solution 7) containing a photoinitiator and a dyestuff wasadded to 100 parts by weight of the above-prepared metal compoundsolution to give a composition in a homogeneous solution state foroptical recording.

<Solution 7> HPA 3 g BTTB 0.4 g KCD 0.01 g Methylene chloride 1.5 gMethanol 0.5 g

The solution 7 was prepared as follows. BTTB and the KCD were dissolvedin a mixture of methylene chloride with methanol to prepare a solution,and the photopolymerizable monomer was added to the solution to obtain ahomogeneous solution 7.

A photosensitive material was prepared using the above-prepared solutionof the composition for optical recording, and exposed, in the samemanner as in Example 1, to produce a transmission type hologram(grating). When the spatial frequency was 1,400 line pairs/mm, theso-produced hologram showed a diffraction efficiency of about 35% afterthe exposure of the entire surface and a diffraction efficiency of about40 % after further heat treatment at 100° C. for 1 hour.

The gel (titanium oxide) constituting the hologram had a refractiveindex of 2.4, and the homopolymer of HPA had a refractive index of 1.50.

EXAMPLE 5

A solution 8 having the following composition and a solution 9 havingthe following composition were individually prepared, and the solution 9containing a hydrolysis catalyst was added dropwise to the solution 8with stirring to prepare a homogeneous solution. Then, this solution wasrefluxed at 80° C. for 40 minutes to obtain a metal compound solution.

<Solution 8> TEOS 27 g PDMS 3 g THF 5 cc i-PA 9 cc <Solution 9> i-PA 12cc H₂O 2 cc HCl (concentration: 12N) 5 cc

Under a red safety lamp, a solution 10 having the following compositionwas mixed with 5.0 g of the above metal compound solution to obtain acomposition in a homogeneous solution state for optical recording.

<Solution 10> HPPA 4.75 g EBPA 0.25 g BTTB 0.50 g KCD 0.01 g Methylenechloride/methanol (95/5, weight ratio) 1.00 g

The above-obtained composition was coated on a 300×150×2 mm glasssubstrate with an applicator under a red safety lamp, and the resultantcoating was gelated and dried by allowing the substrate to stand at 30°C. for about 24 hours, to give a crack-free photosensitive layer havinga thickness of about 9.5 μm. Then, a cover film of polyethyleneterephthalate having a thickness of 100 μm was attached onto the abovephotosensitive layer, and the resultant set was cut to a size of 60×60mm to give a photosensitive material composed of a laminate of glasssubstrate-photosensitive layer-polyethylene terephthalate film.

Then, in the optical system shown in FIG. 1, 514.5 nm light oscillatedfrom the argon ion laser was projected on each of the threephotosensitive materials (laminate of glass substrate-photosensitivelayer-polyethylene terephthalate film) at an angle of θ to expose eachphotosensitive material to interference fringes in the same manner as inExample 1. The values of the angle θ were respectively 5°, 14° and 42°,and the exposure energy was 30 to 50 mJ/cm².

After the holographic exposures, the entire surface of eachphotosensitive material was exposed to a 30 W fluorescent lamppositioned at a distance of 3 cm for about 15 minutes to complete thepolymerization of unpolymerized monomer and fix it.

Gratings were produced in each of the so-obtained photosensitivematerials by means of interference fringes having a spatial frequency ofabout 170, 480 or 1,400 line pairs/mm.

The polyethylene terephthalate film was peeled off from each grating,and the gratings were heated up to 500° C. in an electric furnace at atemperature elevation rate of 50° C./hour. The gratings were kept atthis temperature for 4 hours. Then, the temperature inside the electricfurnace was gradually decreased to room temperature in about 10 hours,and the gratings were taken out to give laminates each of which wascomposed of a 1.2 μm thick SiO₂ film where the grating having a spatialfrequency of 170, 480 or 1,400 line pairs/mm was recorded and the glasssubstrate. The cross section of each SiO₂ film was observed through anelectron microscope at a magnification of 1,000× to 50,000× to show theformation of a surface concavo-convex form based on the intensity of theinterference fringes and a slight density distribution of silicaparticles. That is, the mountain range-like convex portion on thesurface had a height of about 1 μm and a width of about 1 μm, and thepitch of the concavo-convex form (i.e., distance between the center ofone convex portion and the center of a neighboring convex portion) wasabout 6 μm, about 2 μm or about 0.7 μm depending upon the above spatialfrequency. It was also found that silica particles (diameterapproximately 0.01-0.1 μm) were densely present inside convex portionson the film surface and they were coarsely present (there were manypores) in concave portions. The function of the film as a grating wasmainly based on the concavo-convex form on the surface.

EXAMPLE 6

A USAF test target (spatial frequency 1-228 line pairs/mm, supplied byMelles Griot) as a masking sheet was placed on the same photosensitivematerial as that prepared in Example 5, and the photosensitive materialwas exposed to a 2 kW ultraviolet lamp positioned at a distance of 30 cmfrom the light source for 5 seconds. The polyethylene terephthalate filmwas peeled off, and then the photosensitive material was heated up to500° C. in an electric furnace at a temperature elevation rate of 50°C./hour and kept at 500° C. for 4 hours. The temperature inside theelectric furnace was gradually cooled to room temperature in about 10hours, and the photosensitive material was taken out. The surface of theresultant SiO₂ film was observed through an electron microscope to showthe formation of a surface concavo-convex form based on the maskpattern.

EXAMPLE 7

The following solutions 11 and 12 were individually prepared, and thenthe solution 12 containing a hydrolysis catalyst was gradually addeddropwise to the solution 11 with stirring to obtain a homogeneous metalcompound solution.

<Solution 11> Ti(Oi-Pr)₄ 20 g i-PA 20 cc <Solution 12> i-PA 40 cc H₂O2.0 cc HCl (concentration: 12N) 0.5 cc

Then, under a red safety lamp, a solution 13 having the followingcomposition was mixed with 10.0 g of the above metal compound solutionto obtain a composition in a homogeneous solution state for opticalrecording.

<Solution 13> HPPA 4.75 g EBPA 0.25 g BTTB 0.50 g KCD 0.01 g Methylenechloride/methanol (95/5, weight 1.00 g ratio)

The above composition was coated in the same manner as in Example 5 toobtain a photosensitive material, and grating was recorded by means ofinterference fringes having a spatial frequency of 1,400 line pairs/mm.

The so-obtained optical recording film was measured for diffractionefficiency with 632.8 nm beam oscillated from a He—Ne laser. Thediffraction efficiency was calculated as a ratio of the intensity offirst order diffracted light to the intensity of incident light. Table 2shows the results. The optical recording film whose entire surface hadbeen exposed to a fluorescent lamp showed a diffraction efficiency ofonly 2.2%. However, the diffraction efficiency gradually improved as thetemperature for heat treatment increased, and the optical recording filmheat-treated at 400-500° C. showed a diffraction efficiency of about50%.

TABLE 2 Temperature for Diffraction heat treatment efficiency (%) Beforeheating 2.2 (after exposure of entire surface) 100° C. 2.5 200° C. 4.7300° C. 11.8 400° C. 50.5 500° C. 49.2

After the heat treatment at 400° C., the cross section of the opticalrecording film was observed through an electron microscope to show theformation of a pore distribution and a slight surface concavo-convexform as well.

As describe above, the diffraction efficiency of the grating improvesfor the following reason. Before heat treatment, the difference inrefractive index between a region rich with an organic polymer and aregion rich with an inorganic network is small correspondingly to asmall difference (about 0.05) between the refractive index (about 1.55)of a polymer (copolymer of HPPA and EBPA) and the refractive index (1.6)of titanium oxide gel, while in the grating after the heat treatment at400° C., the difference (about 1.35) between the refractive index (about2.4) of titanium oxide and the refractive index (about 1.0) of air isvery large.

What is claimed is:
 1. An optical holographic film, which comprises asilicon dioxide or metal oxide porous gel having a network structure andpores in which gases are present, said porous gel containing a networkportion having a high porosity, a network portion having a low porosityand porosity differences in the network structure, said network portionhaving a high porosity having been formed by exposure to light havinghigh intensity and has a low refractive index, and said network portionhaving a low porosity and having been formed by exposure to light havinglow intensity and has a high refractive index, said film beingsubstantially free from organic compounds.
 2. The film of claim 1, whichhas a refractive index modulated based on the porosity differences inthe network structure.
 3. The film of claim 1, wherein the metal oxideis titanium oxide, zirconium oxide or aluminum oxide.
 4. The film ofclaim 1, wherein the metal oxide is a hydrolysis product or apolycondensation product of organic metal compound.
 5. The film of claim1, which is a hologram.
 6. An optical holographic film which comprises asilicon dioxide or metal oxide porous gel having a network structure andpores in which gases are present, the porous gel having a concavo-convexform on the surface thereof in which the concave portion has been formedby exposure to light having a high intensity and the convex portion hasbeen formed by exposure to light having a low intensity, said film beingsubstantially free from organic compounds.
 7. The film of claim 6, whichhas a phase modulated based on the concavo-convex form on the surface ofthe porous gel or the gel.
 8. The film of claim 6, wherein the metaloxide is titanium oxide, zirconium oxide or aluminum oxide.
 9. The filmof claim 6, wherein the metal oxide is a hydrolysis product or apolycondensation product of organic metal compound.
 10. The film ofclaim 6, which is a hologram.
 11. A process for the production of anoptical holographic film which comprises a silicon dioxide or metaloxide porous gel having a network structure and pores in which gases arepresent, said porous gel containing a network portion having a highporosity, a network portion having a low porosity and porositydifferences in the network structure, said network portion having a highporosity having been formed by exposure to light having high intensityand has a low refractive index, and said network portion having a lowporosity and having been formed by exposure to light having lowintensity and has a high refractive index, said film being substantiallyfree from organic compounds, which process comprises applying acomposition containing a photopolymerizable compound (A), aphotoinitiator (B), a metal compound (C) which is crosslinkable byself-hydrolysis when in contact with water and the subsequentpolycondensation, a solvent (D) for the metal compound, water (E) and acatalyst (F) for the hydrolysis of the metal compound to a substrate toform a coating, drying the coating to form a film for optical recording,irradiating the film with actinic radiation to record optical data, andremoving an organic component contained in the film.
 12. A process forthe production of the optical holographic film which comprises a silicondioxide or metal oxide porous gel having a network structure and poresin which gases are present, the porous gel having a concavo-convex formon the surface thereof in which the concave portion has been formed byexposure to light having a high intensity and the convex portion hasbeen formed by exposure to light having a low intensity, said film beingsubstantially free from organic compounds, which process comprisesapplying a composition containing a photopolymerizable compound (A), aphotoinitiator (B), a metal compound (C) which is crosslinkable byself-hydrolysis when in contact with water and the subsequentpolycondensation, a solvent (D) for the metal compound, water (E) and acatalyst (F) for the hydrolysis of the metal compound to a substrate toform a coating, drying the coating to form a film for optical recording,irradiating the film with actinic radiation to record optical data,removing an organic component contained in the film and heating the filmat a temperature between 200° C. and 1,200° C. for 1 minute to 5 hours.13. A process for producing the optical holographic film which comprisesa silicon dioxide or metal oxide porous gel having a network structureand pores in which gases are present, said porous gel containing anetwork portion having a high porosity, a network portion having a lowporosity and porosity differences in the network structure, said networkportion having a high porosity having been formed by exposure to lighthaving high intensity and has a low refractive index, and said networkportion having a low porosity and having been formed by exposure tolight having low intensity and has a high refractive index, said filmbeing substantially free from organic compounds, said processcomprising: (1) the step of applying a composition containing aphotopolymerizable compound (A), a photoinitiator (B), a metal compound(C) which is crosslinkable by self-hydrolysis when in contact with waterand the subsequent polycondensation, a solvent (D) for the metalcompound, water (E) and a catalyst (F) for the hydrolysis of the metalcompound, the content of the metal compound being 5 to 90 wt %, to asubstrate and removing a volatile component contained in the coating byevaporation to form a film for optical recording in which thephotopolymerizable compound is retained in the network structure of anoxide of the metal formed by the hydrolysis and polycondensation of themetal compound on the substrate; (2) the step of irradiating the filmfor optical recording with actinic radiation to form an area rich withan organic polymer formed by the polymerization of thephotopolymerizable compound in a portion exposed to light having highintensity and an area rich with the oxide of the metal in a portionexposed to light having low intensity of the film; and (3) the step ofremoving an organic component contained in the area rich with theorganic polymer and the area rich with the oxide of the metal in theexposed film to form a film-like network of a metal oxide having a largenumber of pores which are the marks of the removed organic component.14. The process of claim 13, wherein the organic component is removed byheating the optical holographic film at a temperature of 200° C. ormore.
 15. A process for producing an optical holographic film whichcomprises a silicon dioxide or metal oxide porous gel having a networkstructure and pores in which gases are present, the porous gel having aconcavo-convex form on the surface thereof in which the concave portionhas been formed by exposure to light having a high intensity and theconvex portion has been formed by exposure to light having a lowintensity, said film being substantially free from organic compounds,comprising: (1) the step of applying a composition containing aphotopolymerizable compound (A), a photoinitiator (B), a metal compound(C) which is crosslinkable by self-hydrolysis when in contact with waterand the subsequent polycondensation, a solvent (D) for the metalcompound, water (E) and a catalyst (F) for the hydrolysis of the metalcompound, the content of the metal compound being 5 to 90 wt. %, to asubstrate and removing a volatile component contained in the coating byevaporation to form a film for optical recording in which thephotopolymerizable compound is retained in the network structure of anoxide of the metal formed by the hydrolysis and polycondensation of themetal compound on the substrate; (2) the step of irradiating the filmfor optical recording with actinic radiation to form an area rich withan organic polymer formed by the polymerization of thephotopolymerizable compound in a portion exposed to light having highintensity and an area rich with the oxide of the metal in a portionexposed to light having low intensity of the film; and (3) the step ofremoving an organic component contained in the area rich with theorganic polymer and the area rich with the oxide of the metal in theexposed film to form a film-like network of a metal oxide having a largenumber of pores which are the marks of the removed organic component,wherein the organic component is removed at a temperature at which themetal oxide can deform to form a controlled concavo-convex surface onthe optical recording film.
 16. The process of claim 13 or 15 whichcomprises the step of completing the polymerization of thephotopolymerizable compound between the step (2) and the step (3). 17.The process of claim 13 or 15, wherein an interference fringe obtainedby radiation having coherence is used for the irradiation of the actinicradiation.
 18. The process of claim 13 or 15, wherein the actinicradiation is irradiated through a patterning mask placed on the opticalrecording film.
 19. The process of claim 13 or 15, wherein thecomposition contains 10 to 80 wt % of the photopolymerizable compound(A), 0.05 to 30 wt % of the photoinitiator (B), 5 to 90 wt % of themetal compound which is crosslinkable by self-hydrolysis (C), 5 to 90 wt% of the solvent (D), 0.01 to 30 wt % of the water (E) and 0.05 to 30 wt% of the catalyst (F) based on 100 wt % of the total amount of thecomponents (A) to (F).
 20. The process of claim 13 or 15, wherein themetal compound is an alkoxide of silicon, titanium, zirconium oraluminum.